Authorities say this is largely due to the type of oil the tanker was carrying, which is known as condensate.

It tends to evaporate rapidly. Chinese transport authorities say only 1% of the condensate oil which has leaked from the stricken tanker remains in the surrounding area.

The Iranian tanker collided with a Hong Kong-registered bulk carrier about 120 nautical miles off the coast of Shanghai on Saturday evening.

Handout photo from January 9, 2018 from China's Transport Ministry shows smoke and flames coming from the burning oil tanker 'Sanchi' off China's east coast. Chinese authorities say no major oil spill has been detected due to the nature of the oil it was carrying. However, 31 sailors remain missing. [Photo: VCG]

The collision sparked a fire, which has created a massive blaze which threatens to sink the ship.

31 of 32 Iranian and Bangladeshi crew onboard the ship are still listed as missing, but presumed dead, with one body only having been recovered so far.